Metabolic diseases are diseases caused by an abnormal metabolic process and may either be congenital due to an inherited enzyme abnormality or acquired due to a disease of an endocrine organ or failure of a metabolically important organ such as the liver or the pancreas.
Diabetes mellitus is a disease state or process derived from multiple causative factors and is defined as a chronic hyperglycemia associated with resulting damages to organs and dysfunctions of metabolic processes. Depending on its etiology, one differentiates between several forms of diabetes, which are either due to an absolute (lacking or decreased insulin secretion) or to a relative lack of insulin. Diabetes mellitus Type I (IDDM, insulin-dependent diabetes mellitus) generally occurs in adolescents under 20 years of age. It is assumed to be of auto-immune etiology, leading to an insulitis with the subsequent destruction of the beta cells of the islets of Langerhans which are responsible for the insulin synthesis. In addition, in latent autoimmune diabetes in adults (LADA; Diabetes Care. 8: 1460-1467, 2001) beta cells are being destroyed due to autoimmune attack. The amount of insulin produced by the remaining pancreatic islet cells is too low, resulting in elevated blood glucose levels (hyperglycemia). Diabetes mellitus Type II generally occurs at an older age. It is above all associated with a resistance to insulin in the liver and the skeletal muscles, but also with a defect of the islets of Langerhans. High blood glucose levels (and also high blood lipid levels) in turn lead to an impairment of beta cell function and to an increase in beta cell apoptosis.
Persistent or inadequately controlled hyperglycemia is associated with a wide range of pathologies. Diabetes is a very disabling disease, because today's common antidiabetic drugs do not control blood sugar levels well enough to completely prevent the occurrence of high and low blood sugar levels. Out of range blood sugar levels are toxic and cause long-term complications for example retinopathy, nephropathy, neuropathy and peripheral vascular disease. There is also a host of related conditions, such as obesity, hypertension, stroke, heart disease and hyperlipidemia, for which persons with diabetes are substantially at risk.
Obesity is associated with an increased risk of follow-up diseases such as cardiovascular diseases, hypertension, diabetes, hyperlipidemia and an increased mortality. Diabetes (insulin resistance) and obesity are part of the “metabolic syndrome” which is defined as the linkage between several diseases (also referred to as syndrome X, insulin-resistance syndrome, or deadly quartet). These often occur in the same patients and are major risk factors for development of diabetes type II and cardiovascular disease. It has been suggested that the control of lipid levels and glucose levels is required to treat diabetes type II, heart disease, and other occurrences of metabolic syndrome (see e.g., Diabetes 48: 1836-1841, 1999; JAMA 288: 2209-2716, 2002).
Sensing and regulating cellular the energy status in response to environmental and/or nutritional stress is highly important and AMP-activated protein kinase (AMPK) is a major contributor for this task (Hardie et al. (2001) Bioessays 23: 1112; Kemp et al. (2003) Biochem. Soc. Transactions 31: 162). Cellular energy depletion leads to the activation of AMP-activated protein kinase (AMPK) thereby inhibiting ATP consuming and upregulating ATP generating pathways. On a cellular level several substrates are regulated by AMP-activated protein kinase (AMPK) such as acetyl-CoA-carboxylase (ACC) and HMG-CoA-reductase (Carling et al. (1987) FEBS Letters 223: 217), hormone-sensitive lipase (Garton et al. (1989) Eur. J. Biochem. 179: 249), malonyl-CoA-decarboxylase (Saha et al. (2000) J. Biol. Chem. 275: 24279) and glycerol-3-phosphate acyltransferase (Muoio et al. (1999) Biochem. J. 338: 783).
AMP-activated protein kinase (AMPK) mediated phosphorylation of ACC leads to inhibition of ACC, which then results in a decrease of fatty acid synthesis while fatty acid oxidation is increased. AMP-activated protein kinase (AMPK) mediated phosphorylation and inhibition of HMG-CoA-reductase leads to a decrease in cholesterol synthesis. Triacylglycerol synthesis and fatty acid oxidation is regulated by AMP-activated protein kinase (AMPK) via glycerol-3-phosphate acyltransferase. In addition AMP-activated protein kinase (AMPK) stimulates glucose transport in skeletal muscle and regulates the expression of genes involved in fatty acid and glucose metabolism (Hardie et al. (2001) Bioessays 23: 1112; Kemp et al. (2003) Biochem. Soc. Transactions 31: 162). Glucose homeostasis is mediated in liver and muscle by AMP-activated protein kinase (AMPK), wherein activation of AMP-activated protein kinase (AMPK) leads to an increase in GLUT 4-dependent glucose uptake (Sakamoto et al. (2008) Am. J. Physiol. Endocrinol. Metab. 295: E29-E37; Karagounis et al. (2009) Int. J. Biochem. Cell Biol. 41: 2360-2363; Pehmoller et al. (2009) Am. J. Physiol. Endocrinol. Metab. 297: E665-E675).
Besides energy regulation on a cellular level AMP-activated protein kinase (AMPK) also regulates whole body energy metabolism. Independently of the cellular AMP level AMP-activated protein kinase (AMPK) can be activated by the adipocyte derived hormones leptin (Minokoski et al. (2002) Nature 415: 339) and adiponectin (Yamauchi et al. (2002) Nature Medicine 8: 1288).
From the points discussed above activation of AMP-activated protein kinase (AMPK) in vivo is expected to result in hepatic stimulation of fatty acid oxidation; inhibition of cholesterol synthesis, lipogenesis and triglyceride synthesis; stimulation of skeletal muscle fatty acid oxidation and glucose uptake; improved insulin action; increase in energy expenditure and hence a decrease in body weight.